Exhaust treating system for lean-burn CNG engine

Abstract

A three-way catalyst system for efficiently converting the exhaust gas from a CNG-fueled engine when operated under lean conditions at a redox ratio of 0.02-0.9, the system comprising a first stage catalyst comprising a transition metal-containing zeolite; means for injecting a hydrocarbon into said exhaust gas prior to entry of said exhaust gas into said first stage catalyst, said hydrocarbon having a greater affinity than CH 4 in its ability to react with NO; and a second stage catalyst for treating the effluent from said first stage catalyst and comprising a high surface area alumina impregnated with discontinuous La 2 O 3 and palladium. The invention also comprehends a method of treating exhaust gases from a lean-burn CNG-fueled engine, operating at a redox ratio of 0.02-0.9; exposing such exhaust gases to a first stage catalyst consisting of copper-ZSM5 zeolite having at least 3% by weight ion-exchange copper; injecting a fast-acting hydrocarbon into said exhaust gas prior to entry of the exhaust gas into said first stage catalyst, the hydrocarbon having a greater affinity than CH 4 in its ability to react with NO; and exposing the effluent from the first stage catalyst to a second stage catalyst comprising palladium supported on alumina containing discontinuous La 2 O 3 .

We claim: 
     
       1. A three-way catalyst system for efficiently converting the exhaust gas from a CNG-fueled internal combustion engine when operated at lean conditions defined by a redox ratio of 0.02-0.9, the system comprising: (a) a first stage catalyst comprising a transition metal-containing zeolite;   (b) means for injecting a fast-burning hydrocarbon into said exhaust prior to entry of said exhaust gas into said first stage catalyst, said hydrocarbon having a greater affinity than CH 4  in its ability to react with NO; and   (c) a second stage catalyst for treating the effluent from said first stage catalyst and comprising alumina impregnated with palladium.   
     
     
       2. The system as in claim 1, in which said second stage catalyst comprises also La 2  O 3 . 
     
     
       3. The system as in claim 1, in which said second stage catalyst comprises gamma alumina impregnated with 0.5-20% La 2  O 3  and 0.2-30% palladium. 
     
     
       4. The system as in claim 2, in which said lanthana is discontinuously coated on said alumina. 
     
     
       5. The system as in claim 1, in which said first stage catalyst is a high silica zeolite supported on a high surface area alumina substrate and in which said transition metal is selected from the group consisting of copper, cobalt, nickel, chromium, iron, manganese, silver, zinc, calcium, and compatible mixtures thereof. 
     
     
       6. The system as in claim 1, in which said first stage catalyst is copper ion-exchange zeolite containing at least 3% by weight copper. 
     
     
       7. The system as in claim 1, in which said second stage catalyst contains palladium in a crystalline form having a crystal size in the range of 5-500 angstroms and the palladium/lanthanum ratio is in the range of 0.1-60.